Use oslo function for parsing bool from env var

The tests were previously defining their own way of parsing booleans
from strings.  This change switches to using a function defined in
the oslo.utils.strutils module.

Implements: bp retargetable-functional-testing

Change-Id: Ic06de1ed67dd9cc762415d362a928a89da0ce2f0
Merge "Use oslo function for parsing bool from env var"